1. Field of the Invention
The present invention relates to a visibility measuring apparatus, and more particularly to such apparatus using at least one transmitter for emission of light signals, a receiver optic for each measuring base for receiving emitted light signals, and a central unit for controlling the light emission and for processing of the signals received from the receiver optic.
2. Prior Art
A visibility measuring apparatus disclosed in British Patent Specification No. 1,497,214 discloses optical mirrors or prisms reflecting the light signals emitted from a common transmitter to a central unit integrated therewith. The central unit receives and detects the reflected signals and determines the travel delay time of the signal. The distance along which a light signal passes from the transmitter to the receiver located in the central unit for evaluating the signal is twice the measuring base. The measuring base is defined as the distance between the transmitter and the reflecting mirrors or prisms. This means that the light signal arriving at the receiver has been damped to such an extent as to correspond to the amount of free-floating water droplets, other particles, or air pollution present in the measuring base or at the transmitter or receiver, which deteriorates the visibility along a distance equal to twice the measuring base.
The mode of operation of such known visibility measuring means is as follows. If a strong, local bank of mist appears, for example, at the center of the longest measuring base, the measuring means indicates that the visibility range is equal to the distance to the bank of mist, irrespective of the visibility conditions behind the mist. Additionally, local pollution of any of the light-reflecting members may cause incorrect indication of the measured visibility conditions.